Steve Lynch, congressman and former president of Ironworkers Union Local #7, abandoned plans to run for US Senate in 2009 when labor leaders proved reluctant to commit to him. Barely three years later, a long and growing list of those same organization have endorsed Lynch, and are fueling his primary campaign.

The turnaround is striking; it is also the main reason Ed Markey is not a shoo-in for the Democratic nomination.

The importance of labor backing was in evidence leading up to this past Friday, when the state AFL-CIO executive board gathered to hear from both candidates. Labor leaders say that both camps worked hard to sway the vote, which in the end saw neither side win the necessary two-thirds for the endorsement.

These endorsements are about more than a line on a campaign website, or contributions, or even the potential votes of thousands of the union's members.

It's about volunteers on the phones, knocking doors, attending rallies, and organizing get-out-the-vote efforts — particularly critical in this short-sprint, low-interest, and low-budget special election.

MORE THAN ISSUES

Lynch and Markey have nearly identical, and nearly perfect, pro-union voting records. "We have two good friends of labor in the race," Steven Tolman, president of the Massachusetts AFL-CIO told me last week, before the union vote.

But Lynch has a clear leg up on Markey. Or almost anyone, for that matter. Labor leaders in the state, including Tolman, refer to Lynch as a brother, or part of the family.

When labor failed to support Lynch for US Senate in 2009, media reports pinned it on his opposition to the Affordable Care Act (ACA). Hence the general surprise at labor's amnesia in embracing Lynch now.

There was more to the 2009 rejection than that, however. Some locals were simply playing safe by getting on board with the frontrunner. And Coakley had been piling up chits with local unions by aggressively pursuing rule-skirting employers they brought to her office's attention — while Lynch was off meeting with national union lobbyists on more remote issues.

In fact, the unions who were most furious about Lynch's ACA vote are simply too liberal — on non-labor issues — to ever warm up to him. That includes teachers' unions, which are backing Markey; and the Service Employees International Union (SEIU), whose members skew heavily female, black, and Hispanic, and which is likely to endorse Markey next week.

As one SEIU insider put it to me, "Do we have a position on choice? No. Would our members kill us [if we endorsed a pro-life candidate]? Yes."

The building trades, by contrast, consist mostly of white men for whom Markey is far too liberal.

But Lynch has won the endorsement of the women-dominated Massachusetts Nurses Association (MNA). That group surprised many by endorsing Elizabeth Warren before she even announced herself a candidate — but had an equally easy choice backing the less-liberal Lynch this time, says MNA president Donna Kelly-Williams. "Congressman Lynch has been a strong partner on our top legislative priority" — requiring higher RN-to-patient hospital staffing levels — and "Markey did not sign on to the legislation."

MAY FLOWERS?

This divide among unions has some Democrats concerned — especially if Markey wins the April 30 primary. Members of the locals that backed Lynch, having spent two months demonizing Markey, could flip right across the partisan line to vote for the Republican nominee — as so many of them did for Scott Brown.

Not so, insists Marty Walsh, state representative from Dorchester and president of the Boston Building Trades Council. "On May first, labor will have a united front," he says.

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